The Tropical Tropopause Layer (TTL) is an important region forstratospheric chemistry and tropospheric climate. An overview of the TTLand critical processes is presented, with some recent work on simulatedchanges to the TTL. The TTL is the source region for most stratosphericair, and sets the 'boundary condition' for the stratosphere. Transportpathways may regulate how species get into the stratosphere. In addition,the TTL itself is an important region for climate: clouds and humidity in theTTL are important for the tropospheric radiation balance, and alterationsthat occur to that may directly change surface climate. Finally the TTLstructure may be an indicator of climate change. Thus correctly simulatingTTL processes are important for simulating climate change and ozonedepletion and recovery. A detailed analysis of the TTL in CoupledChemistry-Climate Models is presented using multi-model ensemblesassembled for the WMO Ozone assessments. A detailed analysis ofsimulated climatology of the TTL indicates that models are able tosimulated the TTL structure. Comparisons across models indicate thatlarge scale advection and dynamics, not details of convective transportare critical for the TTL. A multi-model ensemble is used to examine pastand future trends in the TTL, and expectations for the 21st century andconfidence in those trends are assessed. Implications for climate andcirculation trends are analyzed.